Process and device for detecting revolutions of a rotating element by twice fourier transform

ABSTRACT

In a process and a device for measuring the rate of rotation of a rotating part which is completely enclosed by a housing, the current signal which is generated by the rotating part is subjected to two Fourier transforms after corresponding signal processing in digital form. The rate of rotation is determined from the spectrum obtained after the second Fourier transform. The rate of rotation is determined from the time pertaining to the maximum of the spectrum which is directly proportional to the rate of rotation.

BACKGROUND OF THE INVENTION

The present invention relates to a process and a device for detectingrevolution of a rotating element, and in particular to such a processand device in which the rate of rotation of a rotating part enclosed bya housing is measured.

To determine the rate of rotation of rotating parts, markings which aregenerally arranged at the rotating part are sensed by an inductivetransmitter or an optical sensor. When such marks pass by the stationarytransmitter, voltage pulses are generated, the rate of rotation beingcalculated from the repetition rate of these voltage pulses.

It is not possible to calculate the rate of rotation in this way in thecase of rotating parts which are completely enclosed by a housing. Forexample, in electric fuel pumps which are completely encapsulated, norotatable parts are guided outward which would make it possible todetermine the rate of rotation in a simple manner. For this reason,signals dependent on the rate of rotation have been analyzed inlaboratory tests for determining the rate of rotation, e.g.: theoscillation acceleration on the electric fuel pump housing, fluctuationsin pressure on the suction side or the pressure side, fluctuations incurrent in the electric supply lines, fluctuations in the magnetic fieldoutside the electric fuel pumps which are caused by the rotatingarmature segments with the respective current-carrying alternations.

These signals were determined, for example, by separate sensors andprocessed in a subsequent evaluating arrangement. Determining the rateof rotation by measuring the intervals between individual oscillationshas not proven reliable in the above-mentioned laboratory tests.

ADVANTAGES OF THE INVENTION

The process according to the invention with its characterizing featureshas the advantage over the prior art that a definite and precisemeasurement of the rate of rotation is made possible by determining asignal dependent on the rate of rotation and subsequently subjecting ittwice to a Fourier transform.

It is particularly advantageous to use the current signal as anadditional signal dependent on the rate of rotation in connection with ameasurement of the rate of rotation of the electric fuel pump and tosubject it to two fast Fourier transforms. The alternating supply ofcurrent to the armature windings during the rotation of the armaturecauses fluctuations in current. These current fluctuations, which occurwith an exactly determined frequency which is dependent on the rate ofrotation, are detected as maxima in the spectrum after a first Fouriertransform; after an additional Fourier transform, the rate of rotationoccurs as an absolute maximum and can be evaluated in a simple manner.

A plausibility check is made possible by comparing the rate of rotationdetermined from the spectrum obtained by a second Fourier transform,e.g. with the main components of the spectrum obtained after the firstFourier transform.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a device for detecting revolutions of arotating element in accordance with the present invention;

FIG. 1a shows an internal gear pump schematically;

FIG. 2 shows the measured current curve over time subsequent to bandpassfiltering;

FIG. 3 shows the spectrum obtained after the first Fourier transform asa function of the frequency; and

FIG. 4 shows the spectrum obtained after another Fourier transform overtime.

DESCRIPTION A PREFERRED EMBODIMENT

FIG. 1 shows an embodiment example of the invention in the form of ablock wiring diagram. The electric fuel pump whose rate of rotation isto be determined is designated by 10. A measuring device which measuresthe current signal or the magnetic fields of the electric fuel pumpcaused by the current signal is designated by 11, the measured currentbeing transformed simultaneously into voltage U1.

An adaptive amplifier 12 (having a variable amplification factoradaptable to specific conditions) in which the voltage U1 is amplifiedis connected to the measuring device 11. The signal is filtered in asubsequent bandpass filter 13 so that a signal U2 is made available atthe output of the bandpass filter for further evaluation. The signal U2is plotted over time t in FIG. 2.

The signal U2 is digitized in the analog-to-digital converter 14.Fourier transforms run in a subsequent block 15 which can be a componentof a computer 16; 15 can also be designated as a FFT card.

The measurements of the rate of rotation determined in the computer 16are read out in an output unit designated by 17.

The electric fuel pump 10 whose rate of rotation is to be determined canbe any electric pump, e.g. a roller cell pump, an internal gear pump, aperipheral pump or a side-channel pump. All of these types of pump arecompletely enclosed by a housing so that no rotatable part by which therate of rotation could be determined relatively easily can be accessedor seen from the outside. Of course, the invention can be extended toall coupled motors.

The electric current fed to the electric fuel pump 10 which ultimatelycauses the armature of the electric fuel pump to rotate is to beevaluated according to the invention.

The alternating current supply to the armature windings of the pumpsduring rotation causes periodic fluctuations in current. These currentfluctuations are dependent upon the rate of rotation and can be utilizedfor determining the rate of rotation of the electric fuel pump bymeasuring the so-called current shape picture I.

In an electric fuel pump 10 shown in FIG. 1a, an internal gear pump withan inlet A and an outlet B with eight-slot armature 18, the firstrepetition follows after eight commutations.

The alternating frequency occurring in the current alternation as wellas integral multiples of this alternating frequency and the frequency ofthe rate of rotation are present in the current shape picture I.

The current of the electric fuel pump is measured in the measuringdevice 11 in that the current is directed e.g. through a shunt resistorand the voltage U1 dropping across this resistor is tapped.

An improved measuring device 11 uses a current transformer which isbased on the principle of compensation of magnetic fields and supplies ameasurement current proportional to the primary current. This current istransformed via a precision metal-film resistor into a proportionalvoltage U1 which is subsequently evaluated.

Since the voltage U1 representing the input voltage of the adaptiveamplifier 12 can fluctuate within a wide range, it is amplified in theadaptive amplifier 12 in such a way that an output voltage occurs withinfixed limits at its output regardless of the level of input voltage.Moreover, the voltage U1 contains the same frequency components as thecurrent picture I.

The bandpass filter 13 is a conventional bandpass filter which passesthe frequency components to be studied and absorbs the rest of thefrequency components.

The signal U2 is digitized in the analog-to-digital converter 14 so thata digital Fourier transform can be carried out subsequently.

The FFT card 15 is a device in which fast Fourier transforms (FFT=fastFourier transform) are calculated. Such FFT's are known e.g. from J. W.Cooley and J. W. Tinkey: "An algorithm for the machine calculation ofcomplex fourier series" in Math. of Comp., Vol. 19, No. 90, pp. 297-301,1965.

The evaluation of the signals or spectra obtained after the Fouriertransform or transforms is effected in a computer 16 which is e.g. apersonal computer or a single-board computer.

When the signal U2(t) shown in FIG. 2, which is obtained from thecurrent signal of the electric fuel pump after adaptive amplificationand bandpass filtering, is subjected to a first Fourier transform, thespectrum U3 (f) occurs. This spectrum contains a plurality of relativemaxima at frequencies corresponding to a multiple of the rate ofrotation of the electric fuel pump. It would be possible to determinethe rate of rotation from these maxima, but it has been shown that sucha determination of the rate of rotation is destroyed by various effects.On the other hand, if the signal U3(f) is subjected a second time to aFFT in 15, a signal U4 (t) occurs which can be directly evaluated. In sodoing, the periodicity of the spectrum harmonically generated by manyrevolutions is made use of. No incorrect measurements are made whenindividual harmonics are absent from the spectrum or the peak amplitudesfluctuate sharply from one spectrum to another.

In the spectrum U4 (t) shown in FIG. 4, the rate of rotation occurs asan absolute maximum x because the corresponding line interval is theclearest periodicity in the FFT spectrum. The position designated by xin FIG. 4 shows the maximum which is dependent on the rate of rotation;the respective time t_(x) is determined and can be used, by way ofobserving the proportionality, for exactly determining the rate ofrotation, since n˜1/t.

The low maxima contained in FIG. 4 denote times which correspond totwice, triple or quadruple the rate of rotation.

A comparison of the rate of rotation determined from the spectrumaccording to FIG. 4 with the values of the rate of rotation derivablefrom FIG. 3 enables a plausibility check to be effected in many ways bycomparison with the main components of the FFT spectrum.

The accuracy of measurement of the described process depends on the datalength of the calculations of the Fourier transforms. If exactmeasurement results are to be obtained, an increased expenditure oncomputing means is required when carrying out the Fourier transforms.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofprocesses and constructions differing from the types described above.

While the invention has been illustrated and described as embodied in aprocess and a device for measuring a rate of rotation of a rotatingpart, it is not intended to be limited to the details shown, sincevarious modifications and structural changes may be made withoutdeparting in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.
 1. A process for measuring a rate ofrotation of a rotating part which is enclosed by a housing, comprisingthe steps of determining a signal dependent on a rate of rotation of arotating part; filtering and digitizing the signal and subjected thesignal twice to a Fourier transform so as to form a spectrum; anddetermining a rate of rotation from the spectrum obtained by thepreceding steps, by evaluating an absolute maximum of the spectrum.
 2. Aprocess as defined in claim 1, wherein said determining the rate ofrotation includes determining the rate of rotation by evaluating areciprocal value of times between maxima of the spectrum.
 3. A processas defined in claim 1, wherein said determining a signal of the rotatingpart includes determining a signal of the rotating part formed as anarmature of an electric fuel pump.
 4. A process as defined in claim 3,wherein said signal is a current signal of the electric fuel pump.
 5. Aprocess as defined in claim 3, wherein said signal is a magnetic fieldsignal of the electric fuel pump.
 6. A process as defined in claim 1,wherein said subjection of the signal twice to a Fourier transformincludes effecting the subjection in a microcomputer.
 7. A process asdefined in claim 1, wherein said evaluating includes an evaluatingeffected in a microcomputer.
 8. A process as defined in claim 1, whereinsaid subjection to a Fourier transform and evaluating the absolutemaximum are effected in a microcomputer.
 9. A device for measuring arate of rotation of a rotating part which is enclosed by a housing,comprising measuring means for determining a signal which is dependenton a rate of rotation of a rotating part to produce a signal; filteringmeans operative for filtering the signal; converting means fordigitizing the filtered signal; a Fourier analyzer for carrying outtwice a Fourier transform of the filtered and digitized signal so as toform a spectrum; and computer means for determining the rate of rotationfrom the spectrum obtained by the preceding steps, by evaluating anabsolute maximum of the spectrum.
 10. A device as defined in claim 9,wherein said filtering means is a bandpass filter; and furthercomprising an amplifier connecting said measuring means with saidbandpass filter so that the signal is first amplified and then filteredin said bandpass filter.
 11. A device as defined in claim 9, whereinsaid converting means is an analog-to-digital converter.
 12. A device asdefined in claim 10, wherein said amplifier is an adaptive amplifier.13. A device as defined in claim 9, wherein said measuring meansincludes a current measuring device operating on the principle ofcompensation of magnetic fields and supplying at its output the signalwhich is a voltage corresponding to a measured current.